Cold-formed spring having high fatigue strength and high corrosion fatigue strength, steel for such spring, and method of manufacturing such spring

ABSTRACT

The present invention provides a cold-formed spring having high fatigue strength and high corrosion fatigue strength, a specific type of steel for such a spring, and a method of manufacturing such a cold-formed coil spring. The spring according to the present invention is made from a steel material containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fe substantially constituting the remaining percentage. A wire is produced from the steel, and the wire is subjected to a high-frequency heating process, whereby the wire is hardened at a temperature of 920 to 1040° C. for 5 to 10 seconds, and then tempered at a temperature of 450 to 550° C. for 5 to 20 seconds so that its hardness becomes 50.5 to 53.5 HRC. Finally, the wire undergoes a shot peening process so that its residual stress at 0.2 mm depth from the surface becomes −600 MPa or higher.

[0001] The present invention relates to a cold-formed spring having highfatigue strength, a type of material for such a spring, and a method ofmanufacturing such a spring. More specifically, the present inventionrelates to a cold-formed spring that must have high fatigue strengthagainst corrosive environments, e.g. a suspension spring used inautomobiles, and also relates to a type of material for such a springand a method of manufacturing such a spring.

BACKGROUND OF THE INVENTION

[0002] For the purpose of environmental protection and resourceconservation, it is now demanded that the amount of harmful substancescontained in the exhaust gas emitted from automobiles should be reduced,while it is also desired that automobiles should have better fuelefficiency. To meet such demands, one effective measure is to make thebody of the automobile lighter. Accordingly, efforts have been made tomake every part of the body as light as possible.

[0003] An example of such a body part is the suspension spring, whichwill contribute to the production of a lightweight body if it has ahigher working stress (or design stress). An improvement of the workingstress, however, may cause a problem in respect of the fatigue (ordurability) of the spring.

[0004] Another problem is the corrosion of the spring, which isunavoidable because suspension springs are installed in such locationsof the body that are most badly stained with water or mud. Corrosioncreates pits (or micro-pores) on the surface of the spring, and thesepits serve as the starting point for the fatigue fracture of the spring.

[0005] To address the aforementioned problems, the applicant has filed aJapanese patent application for a “spring having an improved corrosionfatigue resistance”, as disclosed in the Japanese Unexamined PatentPublication No. H 11-241143.

[0006] The aforementioned spring exhibits high durability even underhigh working stress. In the development of this spring, however, it wasassumed that the spring would be hot-formed. If, as in the presentinvention, the spring is used as a cold-formed material, the spring mayhave a poorer durability because of an excessive decarburization (i.e.,a phenomenon in which carbon content escapes from the surface of thespring when the spring is heated at high temperature).

[0007] Accordingly, what remains unsolved is to obtain a specific typeof steel for cold-formed springs that has good durability (or fatigueresistance) as well as good corrosion resistance, and a cold-formed coilspring made from the steel.

[0008] In view of the aforementioned problems, the present inventionintends to provide a cold-formed spring having high fatigue strength(i.e. fatigue resistance or durability) and high corrosion fatiguestrength, a specific type of steel material for such a spring, and amethod of manufacturing such a cold-formed coil spring.

SUMMARY OF THE INVENTION

[0009] To address the aforementioned problems, the present inventionprovides a cold-formed spring having high fatigue strength and highcorrosion fatigue strength, which is made of a wire made from a steelmaterial containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30%of V, with Fe substantially constituting the remaining percentage, andwhich is hardened and tempered by a high-frequency heating process.

[0010] In the aforementioned steel material, it is preferable that thepercentage of P is 0.025% or lower and the percentage of S is 0.020% orlower.

[0011] It is also preferable that the wire has the tensile strength of1800 to 2000 MPa and a reduction of area of 35% or higher after beinghardened and tempered by the high-frequency heating process.

[0012] It is also preferable that the wire has a hardness of 50.5 to53.5 HRC after being hardened and tempered, and the spring is subject toa shot peening process so that the residual stress at 0.2 mm depth fromthe surface becomes −600 MPa or higher.

[0013] The present invention also provides a method of manufacturing acoil spring having high fatigue strength and high corrosion fatiguestrength, in which the spring is made from a steel material containing,in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V, with Fesubstantially constituting the remaining percentage, and which includesthe steps of making a wire from the steel material, hardening andtempering the wire by a high-frequency heating process and cold-coilingthe wire into the spring.

[0014] It is preferable that the high-frequency heating process includesthe steps of hardening the wire at a temperature of 920 to 1040° C. for5 to 20 seconds, rapidly cooling the wire, and tempering the wire at atemperature of 450 to 550° C. for 5 to 20 seconds. More preferably, thehardening temperature is within the range from 940 to 1020° C. and thetempering temperature is within the range from 480 to 520° C.

[0015] It is also preferable that the wire is rapidly cooled after beingtempered.

[0016] The present invention also provides a type of steel material forcold-forming a spring hardened and tempered by a high-frequency heatingprocess, which contains, in weight percentage, 0.45 to 0.52% of C, 1.80to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to0.30% of V, with Fe substantially constituting the remaining percentage.

[0017] In the steel material, it is preferable that the percentage of Pis 0.025% or lower and the percentage of S is 0.020% or lower.

[0018] For the cold-formed spring having high fatigue strength and highcorrosion fatigue strength according to the present invention, thepercentage ranges of the elements of the steel material have beenspecified on the basis of the following reasons.

[0019] Carbon (C): 0.45 to 0.52%

[0020] Carbon has the greatest influence on the strength of the steelmaterial, and any steel material for suspension spring must contain0.45% or more of carbon to have such a strength that provides anadequate durability (or fatigue resistance). However, when the carboncontent is higher than 0.52%, the corrosion fatigue strength willdeteriorate due to the decrease in the toughness of the material.

[0021] Silicon (Si): 1.80 to 2.00%

[0022] Similar to carbon, silicon increases the strength of the steelmaterial. Also, in the case of manufacturing a spring, silicon is animportant element to increase the sag resistance of the spring. Undernormal working conditions for automobiles, the sag of the spring will benoticeable when the silicon content is lower than 0.18%, which maydecrease the height of the body. Silicon also promotes the surfacedecarburization during the heating process. For a spring that ismaximally loaded on its surface when used, the decarburization must beprimarily considered. When the silicon content is higher than 2.0%, thedecarburization will be noticeable during the heating process forhardening. For this reason, the present invention has set the upperlimit of the silicon content at 2.0%.

[0023] Nickel (Ni): 0.30 to 0.80%

[0024] Nickel improves the corrosion resistance of the steel material.In the case of a suspension spring, the nickel content must be 0.30% orhigher to provide an adequate corrosion resistance. Use of more than0.80% of nickel, however, is not recommendable because it provides noimprovement of the corrosion resistance, which is saturated at 0.80%,while it unnecessarily increases the manufacturing cost due to nickelbeing an expensive element.

[0025] Chromium (Cr): 0.15 to 0.35%

[0026] Similar to nickel, chromium improves the corrosion resistance ofthe steel material. Furthermore, chromium improves the hardening effect.To provide the steel material with adequate strength, toughness anddurability, the heating process must be fully performed. Therefore, thespring must be completely hardened to its core. For this purpose, thesteel material according to the present invention contains 0.15% or moreof chromium. However, with respect to the diameter of the suspensionspring that the present invention concerns, 0.35% of chromium provides asufficient hardening effect. Percentages higher than that willundesirably increase the residual austenite.

[0027] Vanadium (V): 0.15 to 0.30%

[0028] Vanadium precipitates in the form of fine particles of carbideinside the steel material, which prevents the development of crystalgrains during the heating process. The reduction of the grain size iseffective in improving the corrosion fatigue resistance as well as thetoughness of the steel material. To obtain such effects, the vanadiumcontent must be 0.15% or higher. The percentage, however, needs to be0.30% or lower because percentages higher than that are likely topromote the development of each vanadium carbide particle rather thanincrease the precipitation sites of the carbide. The development ofvanadium carbide particles may decrease the toughness and the corrosionfatigue resistance.

[0029] Phosphorus (P): 0.025% or lower

[0030] Phosphorus is the first element to precipitate within the grainboundary inside the steel material and deteriorates the strength of thegrain boundary. Since the precipitation of phosphorus decreases thefatigue strength, it is desirable to make the phosphorus content as lowas possible. With respect to the process capability of the manufacturingprocess and the prescribed properties of the spring, the phosphoruscontent should be preferably 0.025% or lower.

[0031] Sulfur (S): 0.020% or lower

[0032] Inside the steel material, sulfur is combined with manganese intoMnS, which is insoluble into the steel material. Since MnS is a softsubstance, it is easily extended through rolling or a similar process,which deteriorates the mechanical properties of the steel material.Therefore, in manufacturing the spring, it is preferable to make thesulfur content as low as possible. With respect to the processcapability of the manufacturing process and the prescribed properties ofthe spring, the sulfur content should be preferably 0.025% or lower.

[0033] A typical process of manufacturing a cold-formed spring includesthe following steps: rolling a material into a wire; changing thediameter of the wire to a predetermined value by drawing or a similarprocess, if necessary; hardening and tempering the wire; coiling thewire into a spring; and conducting the shot peening and the pre-setting.

[0034] The cold-formed spring according to the present invention ismanufactured by using a specific type of steel material whosecomposition satisfies the above-described conditions, and controllingthe hardening and tempering process so that the hardness of the springbecomes 50.5 to 53.5 HRC. When the hardness is lower than this range,the spring cannot have sufficient durability (or fatigue-resistance) tobe used as a suspension spring. When the hardness is higher than therange, the cold-coiling of the wire will be difficult and the coilingprocess will cause some quality damages of the spring, such as a surfaceflaw, surface crack or the deterioration of the toughness due to anexcessive working effect.

[0035] According to the present invention, the hardening and temperingis accomplished by a high-frequency heating process. The high-frequencyheating makes it possible to rapidly raise the temperature and minimizethe surface decarburization. This heating process is also advantageousin that the crystal grains inside the steel material have little time todevelop. Furthermore, this heating process provides a relatively easycontrol of the temperature with good accuracy. These effects areadvantageous particularly for the hardening process. In the temperingprocess, it is also preferable to use a slightly high temperature toshorten the processing time to obtain the same effect (i.e. temperhardness). This preferably improves the sag resistance of the spring.

[0036] For example, it is preferable that, in the high-frequency heatingprocess, the steel material is hardened at a temperature of 920 to 1040°C. (more preferably 940 to 1020° C.) for 5 to 20 seconds, then rapidlycooled, and finally tempered at a temperature of 450 to 550° C. (morepreferably 480 to 520° C.) for 5 to 20 seconds. The temperaturesspecified hereby are higher than in the case of the normal furnaceheating and accordingly shorten the heating time (or heat-up time),whereby the decarburization, the development of the crystal grains andsome other problems are prevented.

[0037] Rapid cooling after the tempering is also recommendable becauseit reduces the unevenness in temper hardness.

[0038] According to the present invention, the conditions for the shotpeening process is regulated so that the residual stress becomes −600MPa or higher at 0.2 mm depth from the surface. Given this level ofcompression residual stress at the surface, the spring will have anadequate durability as a suspension spring. The shot peening may beperformed either under cold temperature (at room temperature) or warmtemperature (at about 250 to 340° C.).

[0039] As described above, the cold-formed spring according to thepresent invention is manufactured by preparing a steel material having aspecific composition and performing a high-frequency heating processunder specific conditions. The spring thus manufactured has goodcorrosion fatigue resistance to be used as a suspension spring. Theappropriate determination of the conditions for the heating, shotpeening and other subsequent processes minimizes the amount of sag thatmay arise when the spring is used.

[0040] Furthermore, the coiling work is facilitated and the qualitydeterioration due to the coiling work is minimized.

[0041] Having the above-described good properties, the cold-formed coilspring according to the present invention can be used under a maximumdesign stress of 1150 MPa or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a table showing the evaluation result of thedecarburization property with respect to the carbon content and thesilicon content.

[0043]FIG. 2 is a graph showing the optimal region of the carbon contentand the silicon content.

[0044]FIG. 3 is a graph showing the relation between the carbon contentand the corrosion durability.

[0045]FIG. 4 is a graph showing the relation between the nickel contentand the weight loss through corrosion.

[0046]FIG. 5 is a graph showing the relation between the vanadiumcontent and the crystal grain size number.

[0047]FIG. 6 is a graph showing the relation between the phosphoruscontent and the corrosion durability.

[0048]FIG. 7 is a graph showing the relation between the tensilestrength and the reduction of area of an example of the steel materialaccording to the present invention and a comparison steel material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] Examples of the present invention are described with reference tothe drawings.

[0050] Twenty pieces of steel samples having different carbon andsilicon contents were prepared for an investigation of thedecarburization property. Each sample was heated for 10 minutes up to900° C., which was then rapidly cooled and cut. The cut surface wasobserved with a microscope, and the sample was evaluated as either “OK(Good)” (when the depth of the perfect (ferrite) decarburization layerwas less than 0.02 mm) or “NG (No Good)” (when the depth was 0.02 mm ormore). The result is shown in FIG. 1.

[0051] From FIG. 1, the optimal region of the carbon content and thesilicon content with respect to the decarburization has been determined,as shown in FIG. 2. The optimal region corresponds to 0.45 to 0.52% ofcarbon content and 1.80 to 2.00% of silicon content, in weightpercentage.

[0052] In FIG. 2, the steel lacks strength within the region with thesilicon content lower than 1.80% and the carbon content lower than0.52%. In this region, the lack of durability causes a considerableamount of sag when the steel is used as a spring. In the region with thesilicon content higher than 2.00%, the decarburization is undesirable.In this region, the surface strength of the steel may significantlydecrease due to the decarburization during the heating process. In theregion with the carbon content higher than 0.52%, the steel lackstoughness. When, as in the case of the suspension spring, the steel isused under a very corrosive environment, the lack of toughness causes adecrease in the durability.

[0053] The second experiment focused on the relation between the carboncontent and the corrosion durability. In this experiment, the loadingcondition was 490±294 MPa. The contents of the principal elements otherthan carbon were as follows: Si: 1.99%, Mn: 0.69%, Ni: 0.55%, Cr: 0.20%and V: 0.20%. The result of this experiment is shown in FIG. 3.

[0054]FIG. 3 shows that the number of cycles to failure in the corrosiondurability test is greater than 50,000 when the carbon content is 0.52%or lower, meaning that the corrosion durability is adequate. When thecarbon content is higher than 0.52%, the number of cycles to failurerapidly decreases to about 30,000 or less.

[0055] The third experiment focused on the relation between the nickelcontent and the corrosion resistance. The contents of the principalelements other than nickel were as follows: C: 0.49%, Si: 1.99%, Mn:0.69%, Cr: 0.20% and V: 0.20%. In the experiment, the process ofspraying a saline solution at the temperature of 35° C. onto the samplefor 3 hours and drying the sample for 21 hours at the temperature of 35°C. was repeated twenty times. Upon completion, the weight loss throughcorrosion per unit surface area (kg/m²) was checked as the criteria forevaluating the corrosion resistance. The result is shown in FIG. 4.

[0056]FIG. 4 shows that the weight loss is 0.4 kg/m² when the nickelcontent is 0.30% or higher, meaning that the corrosion resistance isadequate.

[0057] The fourth experiment focused on the relation between thevanadium content and the grain-refining effect. The contents of theprincipal elements other than vanadium were as follows: C: 0.49%, Si:1.99%, Mn: 0.69%, Ni: 0.55% and Cr: 0.20%. The result is shown in FIG.5.

[0058]FIG. 5 shows that the crystal grain size number is greater than 10when the vanadium content is within the range from 0.15 to 0.30%,meaning that the grain-refining effect is adequate.

[0059] The fifth experiment focused on the relation between thephosphorus content and the corrosion durability. The contents of theprincipal elements other than phosphorus were as follows: C: 0.49%, Si:1.99%, Mn: 0.69%, Ni: 0.55%, Cr: 0.20% and V: 0.20%. The result is shownin FIG. 6.

[0060]FIG. 6 shows that the number of cycles to failure in the corrosiondurability test is greater than 50,000 when the phosphorus content is0.025% or lower, whereas the number decreases to about 20,000 or lesswhen the phosphorus content is higher than 0.025%.

[0061] The sixth experiment focused on the relation between the tensilestrength and the reduction of area of the wire made from a steelmaterial containing 0.49% of C, 1.99% of Si, 0.69% of Mn, 0.55% of Ni,0.20% of Cr and 0.20% of V. The wire was hardened by a high-frequencyheating process and then tempered at various temperatures so that itstensile strength becomes 1800 to 2000 MPa. The relation is shown in FIG.7, which also shows the property data of a conventional steel material(SAE9254) for comparison. The graph in FIG. 7 clearly shows that thesteel material according to the present invention has a higher ductilitythan the conventional one. This result suggests that the presentinvention improves the corrosion fatigue resistance.

1. A cold-formed spring having high fatigue strength and high corrosionfatigue strength, which is made of a wire made from a steel materialcontaining, in weight percentage, 0.45 to 0.52% of C, 1.80 to 2.00% ofSi, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30% of V,with Fe substantially constituting the remaining percentage, and whichis hardened and tempered by a high-frequency heating process.
 2. Thecold-formed spring according to claim 1, wherein the percentage of P is0.025% or lower and the percentage of S is 0.020% or lower.
 3. Thecold-formed spring according to claim 2, wherein the wire has a tensilestrength of 1800 to 2000 MPa and a reduction of area of 35% or higherafter being hardened and tempered by the high-frequency heating process.4. The cold-formed spring according to claim 1, wherein the wire has ahardness of 50.5 to 53.5 HRC after being hardened and tempered, and thespring is subject to a shot peening process so that the residual stressat 0.2 mm depth from the surface becomes −600 MPa or higher.
 5. A methodof manufacturing a coil spring having high fatigue strength and highcorrosion fatigue strength, wherein the spring is made from a steelmaterial containing, in weight percentage, 0.45 to 0.52% of C, 1.80 to2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35% of Cr and 0.15 to 0.30%of V, with Fe substantially constituting the remaining percentage, andthe method comprises the steps of making a wire from the steel material,hardening and tempering the wire by a high-frequency heating process andcold-coiling the wire into the spring.
 6. The method according to claim5, wherein the high-frequency heating process includes the steps ofhardening the wire at a temperature of 920 to 1040° C. for 5 to 20seconds, rapidly cooling the wire, and tempering the wire at atemperature of 450 to 550° C. for 5 to 20 seconds.
 7. The methodaccording to claim 6, wherein the hardening temperature is within therange from 940 to 1020° C. and the tempering temperature is within therange from 480 to 520° C.
 8. The method according to claim 6, whereinthe wire is rapidly cooled after being tempered.
 9. A type of steelmaterial for cold-forming a spring hardened and tempered by ahigh-frequency heating process, containing, in weight percentage, 0.45to 0.52% of C, 1.80 to 2.00% of Si, 0.30 to 0.80% of Ni, 0.15 to 0.35%of Cr and 0.15 to 0.30% of V, with Fe substantially constituting theremaining percentage.
 10. The steel material according to claim 9,wherein the percentage of P is 0.025% or lower and the percentage of Sis 0.020% or lower.
 11. The cold-formed spring according to claim 2wherein the wire has a hardness of 50.5 to 53.5 HRC after being hardenedand tempered, and the spring is subject to a shot peening process sothat the residual stress at 0.2 mm depth from the surface becomes −600MPa or higher.
 12. The cold-formed spring according to claim 3 whereinthe wire has a hardness of 50.5 to 53.5 HRC after being hardened andtempered, and the spring is subject to a shot peening process so thatthe residual stress at 0.2 mm depth from the surface becomes −600 MPa orhigher.